None.
1. Field of the Invention
The present invention generally relates to a flow control actuator capable of exciting a fluid so to produce a high-frequency, pulsed stream. Specifically, the invention excites a fluid via resonance produced within a cavity and edge tones generated along a wedge adjacent to the resonance cavity.
2. Background
The suppression of turbulence is a major challenge in aero-optics, acoustics, and dispensing as each relates to aircraft applications. In aero-optic applications, the integration of a directed energy device onto an aircraft platform requires the attenuation, and preferably elimination, of undesired turbulent structures that degrade both intensity and coherence of a beam projected from an uncovered bay or a covered bay with optically transmissible cover. Acoustic suppression of engine noise is critical to the safe and effective operation of both military and civilian aircraft in urban areas and desirable from a low-observable perspective in military applications. The dispensing of weapons from an open bay or cavity requires the alternation of a pressure field having a negative to-positive gradient along the bay length thereby kicking the tail of the weapon downwards and the nose upwards after release so to cause contact between aircraft and weapon.
The suppression of turbulence is achievable by either low-frequency or high-frequency actuation. Low-frequency actuation techniques are provided in the related arts for turbulence control within cavities. Low-frequency actuation attempts to alter or suppress the growth of spatially evolving, large-scale turbulent structures within the cavity shear layer. This approach is best described as mode competition whereby the primary objective is energy removal from targeted modes. However, low-frequency actuation often results in the redistribution of energy into other modes without overall attenuation. Consequently, low-frequency forcing often dampens one tone thereby failing to suppress other naturally occurring tones. Furthermore, the forcing frequency is quite often close to the natural modes so that when cavity modes drift one or more of the resonant modes are excited creating even higher acoustic loads.
In contrast, high-frequency actuation represents a strategy whereby the shear layer turbulence dissipation mechanism is directly excited by forcing near xe2x80x9cKolmogorovxe2x80x9d -like frequencies thereby suppressing all modes by dissipation of the overall acoustic energy. Large scale vortical structures are replaced by fine-grain turbulence, accompanied by the attenuation of the dynamic load. In cavity applications, fine-grain turbulence attenuates the growth rate of large scale vortical structures and weakens the cavity feedback loop. Whereas in aero-optics applications, it is understood that fine grain turbulence is well with the realm of adaptive optics.
High frequency actuation is achievable via a pulsed airstream. Kibens et al., U.S. Pat. No. 6,375,118 issued Apr. 23, 2002, describes a Hartmann tube attached to a high-pressure (80-120 psi) chamber for attenuating engine noise and reducing turbulence in a cavity. FIG. 1 shows the device described in Kibens et al. whereby air is directly forced into a tube. Kibens"" tube is based on classical designs described by Hutchins et al. in The Modulated Ultrasonic Whistle as an Acoustic Source for Modeling, J. Acoustic Soc. Am. 73(1), January 1983, and later by Kastner et al. in Development and Characterization of Hartmann Tube Based Fluidic Actuators for High Speed Flow Control, Technical Report No. AIAA-2002-0128 presented at the 40th Aerospace Sciences Meeting and Exhibit, Jan 14-17, 2001.
The operating frequency (f) of a Hartmann tube is approximated by the equation
f=c/(4L)
thereby practically constrained by tube length (L) and sound speed (c) of the gas within the tube. The approximation assumes all waves are Mach waves and flow velocity within the tube is negligible. Operating frequencies in the range of 2500 to 5000 Hz are described by Kibens et al.
What is currently required is a high-frequency actuator capable of generating high-quality, high-amplitude, high-frequency tones, while minimizing mass flow rate, for the purpose of attenuating turbulence in aero-optic, acoustic suppression, and weapon dispensing applications. What is required is a device capable of producing high-quality, high amplitude, high-frequency tones exceeding those achievable with a classic Hartmann tube. Furthermore, what is required is a device capable of producing a higher-pressure, pulsed fluid than achievable with a classic Hartmann tube.
An object of the present invention is a device capable of generating a high-quality, high-amplitude, high-frequency tone for the purpose of attenuating turbulence.
A further object of the present invention is a device producing a pulsed fluid stream via a resonance cavity augmented by edge tones.
The present invention is comprised of an inlet port, a stagnation chamber attached to the inlet port thereby providing passage of the fluid into the stagnation chamber, a wedge separating a resonance cavity and an ejector port, and a throat directing fluid from the stagnation chamber across the wedge thereby producing edge tones. The stagnation chamber is pressurized by fluid flow through the inlet port. Fluid from the stagnation chamber passes through a throat and directed across a wedge so that a portion of the fluid flows into the resonance cavity and the remainder through the ejector port exiting the actuator. In preferred embodiments, approximately one-half of the fluid is directed by the wedge into the resonance cavity and the remainder into the ejector port.
The cyclic pressurization and depressurization of the resonance cavity establishes an oscillating shock wave in the resonance tube. In the first half of the oscillation, the resonance cavity is pressurized and a shock is developed traveling into the cavity. In the second half of the oscillation, the shock is reversed so to travel back along the cavity purging the cavity after the shock reaches the opening into the cavity. Back flow from the resonance cavity establishes an unsteady vortex at the corner of the wedge further augmenting frequencies in the fluid expelled from the ejector tube.
Fluid flow over the wedge adjacent to the resonance cavity further augments frequencies within the fluid expelled from the ejector port. A vortex or swirl is created as fluid flows away from the throat in a turbulent fashion and towards the wedge thereby resulting in flow along one side of the wedge. The resulting pressure generated by this flow condition causes the redirection of one or more subsequent vortices thereby resulting in fluid flow along the opposite side of the wedge. This periodic realignment of flow from side-to-side is often referred to as flipping and produces an edge tone. Fluid flipping adjacent to the resonance cavity causes a corresponding variation in pressure which is magnified by resonance when the pressure variation has a component at the resonant frequency of the cavity. The result is a feedback loop of higher pressure, increased flipping, and greater resonance which may lock the fluid frequency to the resonance frequency of the cavity.
Alternate embodiments of the present invention include linear arrangements and circular arrangements of two or more actuators. For example, several actuators may be aligned in a linear fashion with separate inlet ports or a single plenum so to produce a planar flow of excited fluid. Alternately, a plurality of actuators may be arranged to form a single ring with a common plenum or separate inlet ports. Furthermore, a plurality of independently driven actuators may be arranged in a circular fashion so to impinge a column of fluid.
Several advantages are offered by the present invention. The invention provides higher pulse frequencies than achievable with a resonance cavity alone. The invention provides higher pressures within the excited fluid than achievable with a resonance cavity alone. The invention provides tunable control over the frequency characteristics of the excited fluid.